Joined prism assembly and method and apparatus for manufacturing same

ABSTRACT

A joined prism assembly having more than two prisms adhered to each other at joining surfaces is disclosed. The joined prism includes stepped portions between the end faces of the prisms adjacent to the joining surfaces for defining exposed positioning reference surfaces for when the prisms are adhered to each other.

This application is a division of U.S. Ser. No. 07/598,423 filed Oct.10, 1990 U.S. Pat. No. 5,122,217.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a joined (cemented) prism assembly andalso to a method and an apparatus for manufacturing the same.

2. Description of the Related Art

A dichroic prism made of a joined prism assembly (that is, a rectangularparallelepiped prism) is used as a component of a projection displaydevice.

The projection display device is used, for example, as a liquid crystalcolor projector wherein R (red), G (green) and B (blue) illuminatinglight rays are applied to three liquid crystal panels which give R, Gand B color separation information, respectively, to obtain separate R,G and B images which are then combined and projected in the form of anenlarged color image through a projection lens.

FIG. 7 is a conceptual or schematic view of a device which employs adichroic prism and dichroic mirrors for separation and combination(synthesis) of light rays, as an example of a conventional liquidcrystal color projector of the type described above. Liquid crystalpanels 32, 33 and 34 for R, G and B are disposed in opposing relation tothree surfaces (defined as incident surfaces), respectively, of adichroic prism 31 which is in the shape of a rectangular parallelepiped,and a projection lens 35 is disposed so as to face the remaining onesurface (defined as an emergent surface E). A R (Red) reflectingdichroic mirror 32a, a G (green) reflecting dichroic mirror 33a and a B(blue) reflecting dichroic mirror 34a are disposed on the respectiveincidence sides of the liquid crystal panels 32, 33 and 34 at a 45°angle with respect to the associated optical axes. Illuminating lightrays are made incident on these dichroic mirrors 32, 33 and 34. Theilluminating light rays are obtained by passing light from a white lightsource 36 through a collimating condenser lens 37, thereby producingcollimated (parallel) rays of light. The parallel light rays are appliedsuccessively to the B-reflecting dichroic mirror 34a, a total reflectingmirror 38, the G-reflecting dichroic mirror 33a, a total reflectingmirror 39 and the R-reflecting dichroic mirror 32a in this order.

The dichroic prism 31 is provided therein with a B (blue) reflectingfilm 31a and a R (red) reflecting film 31b which are at 45° to thecorresponding optical axes and which are perpendicular to each other.Both the reflecting films 31a and 31b transmit G (green) lighttherethrough.

In the liquid crystal color projector, therefore, when the liquidcrystal panels 32, 33 and 34 are irradiated with parallel light raysproduced by a combination of the white light source 36 and thecollimating lens 37 while these panels 32, 33 and 34 are being switchedon the basis of the corresponding color information, red, green and bluelight rays are respectively transmitted through the liquid crystalpanels 32, 33 and 34 and enter the dichroic prism 31. The incident lightrays are combined (superposed) through the B- and R-reflecting films 31aand 31b provided on the prism 31, and the combined picture image emergesfrom the emergence surface E of the prism 31 and is projected in theform of an enlarged image on a screen 40 through the projection lens 35.

The dichroic prism 31 used in the projection display device consists offour right-angle prisms 31A, each having an isosceles right trianglecross sectional shape. The right angle prisms 31A are joined to eachother so that the adjacent sides of the triangles which define the rightangle apexes are connected to each other. In the dichroic prism, aprecise perpendicularity (squareness) of the joined surfaces of the fourprisms and a strict linearity (i.e., flatness of the B-reflecting film31a and the R-reflecting film 31b) are required, since otherwise adeviation of a projected and superposed image occurs.

However, to realize high perpendicularity and linearity of the joinedsurfaces of the four right prisms 31A which constitute a dichroic prism31, it is necessary to join the four right prisms 31A with the help of amicroscope through which an operator observes. This is, however, a verytroublesome and complicated operation; moreover, it is not efficient.

SUMMARY OF THE INVENTION

The primary object of the present invention is to provide a joined prismassembly which can be simply, quickly and precisely manufactured withless labor.

In a joined prism assembly having more than two prisms according to thepresent invention, a significant feature of the invention resides in theprovision of stepped portions on the ends of the prisms to be joined, sothat a joining end surface is exposed to be used as a reference surfacefor the joining operation.

According to an aspect of the present invention, there is provided ajoined prism assembly having more than two prisms adhered to each otherat joining surfaces. The assembly includes a stepped portion between theend faces of the prisms adjacent to the joining surfaces for definingexposed positioning reference surfaces when the prisms are adhered toeach other.

Preferably, the prisms have different thicknesses, so that thepositioning reference surfaces are defined at the sides of the prismsautomatically.

Alternatively, the prisms can have identical thicknesses. In thealternative, the joining surfaces of the prisms are deviated, in thedirection of the thickness, when they are adhered to each other.

According to another aspect of the present invention, there is provideda method for manufacturing a joined prism assembly having more than twoprisms adhered to each other at joining surfaces thereof. The methodincludes a step of forming a stepped portion between the end faces ofthe prisms adjacent to the joining surfaces for defining exposedpositioning reference surfaces when the prisms are adhered to eachother, and a step of adhering the prisms with the help of the exposedpositioning reference surfaces.

According to still another aspect of the present invention, there isprovided an apparatus for manufacturing a joined prism assembly having aplurality of prism half-assemblies, each having more than one prismadhered to each other at joining surfaces thereof and at least oneexposed reference surface for positioning. The apparatus comprises aprism supporting device for holding one of the prism half-assemblies ata predetermined position, including first reference surfaces with whichouter surfaces of the prism half-assembly can come into contact and atleast one second reference surface with which the exposed referencesurface can come into contact. A positioning device for positioninganother prism half-assembly which is located on and adhered to the prismhalf-assembly held by the prism supporting device is provided thepositioning device is provided with a third reference surface having apredetermined relationship to the second reference surface.

The present invention can be applied to more than two prisms and isparticularly advantageous when applied to a rectangular parallelepipedjoined prism assembly, such as a dichroic prism, which can be used tocombine an image.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described below in detail with reference to theaccompanying drawings showing preferred embodiments of the presentinvention, in which:

FIG. 1 is a front elevational view of a joined prism assembly accordingto the present invention;

FIG. 2 is a plan view of a joined prism assembly according to thepresent invention, as viewed from the direction shown at an arrow II inFIG. 1;

FIG. 3 is a plan view of two triangular prisms joined to each other;

FIG. 4 is a front elevational view of FIG. 3;

FIG. 5 is a plan view of an apparatus for manufacturing a joined prismassembly and prisms joined by the apparatus;

FIG. 6 is a front elevational view of FIG. 5; and,

FIG. 7 is a conceptual schematic view of a liquid crystal display devicewhich employs a dichroic mirror as a joined prism assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The illustrated embodiment is directed to a joined prism assembly in theshape of a square in front elevation, having four triangular prisms.

FIGS. 1 and 2 show a product (that is, a joined prism assembly) havingfour prisms which are completely joined (adhered, cemented) to eachother.

The four right-angle prisms 3 which constitute a rectangularparallelepiped prism 1 have identical isosceles right trianglecross-sections and are adhered to each other at the adjacent side faces(joining faces) 3A, defining the respective right angle apexes.

Among the four right prisms 3, two prisms 3-1 have a thickness t₁ largerthan a thickness t₂ of the remaining two prisms 3-2. The prisms 3-1(referred to as wide prisms) of a larger thickness t₁ and the prisms 3-2(referred to as narrow prisms) of a small thickness t₂ are alternatelyarranged.

The adjacent side faces (that is, joining faces) 3A of the narrow prisms3-2 defining the right angle apexes are symmetrically joined to the sidefaces (joining faces) 3A of the wide prisms 3-1 defining the right angleapexes, so that there are identical stepped portions 5 between the endfaces 3C of the adjacent prisms 3-1 and 3-2. The stepped portions 5define exposed joining end surfaces 3A' which appear in the vicinity ofthe opposite end faces of the prisms 3-1. The exposed joining endsurfaces 3A' have an identical width.

It is not always necessary to adhere the right prisms 3-2 to the rightprisms 3-1 in a strictly symmetrical arrangement. Namely, it is notalways necessary for the exposed joining end surfaces 3A' to have thesame width. This is because the stepped portions 5 are provided topartially expose the joining surfaces 3A of the prisms 3-1 thereby, todefine reference surfaces for the joining operation. The referencesurfaces make it possible to easily join the joining surfaces 3A at aright angle or along a line. Namely, when the wide prisms 3-1 and thenarrow prisms 3-2 are alternately adhered to each other, the joiningsurfaces 3A are partially exposed due to the stepped portions 5 toprovide positioning reference surfaces. When the four prisms 3 areadhered to each other so that the adjacent joining surfaces 3A form aright angle or lie in the same plane, the rectangular parallelepipedprism 1 is obtained. The thicknesses of the prisms 3-1 and 3-2, andparticularly, the thickness t₂ of the prism 3-2 are such that the endportions of the prisms 3-2 corresponding to the stepped portions are notused as a prism in normal use.

Thus, according to the present invention, the rectangular parallelepipedprism 1 can be manufactured with high precision, using a jig whichutilizes the reference surfaces defined by the exposed joining endsurfaces 3A' (or the stepped portions 5) without using an additionaloptical element, such as a microscope, unlike in the prior art.

FIGS. 3 through 6 show a method and an apparatus for joining the prisms,according to the present invention.

First, two right prisms 3-1 having the same thickness t₁ and two rightprisms 3-2 having the same thickness t₂ which is smaller than thethickness t₁ (t₂ <t₁) are prepared. Thereafter, one of the right prisms3-1 and one of the right prisms 3-2 are located on a flat surface 7 insuch a way that the side surfaces 3A thereof lie on the flat surface 7.Thereafter, the other side faces 3A of the two prisms 3-1 and 3-2 areadhered to each other by an adhesive such as a epoxy resin adhesive oran ultraviolet hardening adhesive, etc. (FIG. 4). In this adhesion, theside face 3A of the prism 3-2 having the smaller thickness t₂ is adheredto the center of the corresponding side face 3A of the prism 3-1 toprovide the stepped portions 5 on the opposite sides of the joiningsurfaces 3A, as shown in FIGS. 3 and 4. Consequently, the joining endsurfaces 3A' are exposed.

Two of the same prism half-assemblies 11 shown in FIGS. 3 and 4, eachhaving one prism 3-1 and one prism 3-2. are prepared and adhered to eachother, using a joining device 20, as shown in FIGS. 5 and 6.

The joining device 20 is integrally provided with a base plate 15 whichhas an abutment surface member 17 and a supporting surface member 19which constitutes, together with the abutment surface member 17, a prismassembly supporting means. The base plate 15 is movably attached with amovable support 21 which constitutes a positioning means. The movementof the movable support 21 toward the abutment surface member isrestricted by a stop 23 provided on the base plate 15.

The abutment surface member 17 has an oblique reference 17A as areference surface, and two arms 17B projecting from the opposite sidesof the oblique surface 17A. Each arm 17B has at its front end a verticalreference surface 17C. The arms 17B are spaced from one another at apredetermined distance large enough to insert and remove the right prism3-2, having the small thickness t₂ into and from a space defined betweenthe two arms 17B.

The supporting surface member 19 has an oblique surface 19A, as areference surface, opposing the oblique surface 17A. The obliquesurfaces 17A and 19A form exactly a 45° angle with respect to the baseplate 15 in the opposite directions, respectively, so that the obliquesurfaces 17A and 19A together form a 90° angle. The movable support 21has arms 21B projecting from the opposite upper ends. The arms 21B havevertical reference surfaces 21C. The arms 21B are spaced from oneanother at a predetermined distance large enough to insert and removethe right prism 3-2, having the small thickness t₂ into and from a spacedefined between the two arms 21B.

The vertical reference surfaces 17C and the vertical reference surfaces21C of the movable support 21 form exactly a 90° angle with respect tothe base plate 15 and are formed so that when the movable support 21comes into contact with the stop 23, the vertical reference surfaces 17Cand 21C lie in a vertical plane.

A pair of half-assemblies 11 shown in FIGS. 3 and 4 are adhered to eachother, using the joining apparatus 20 shown in FIGS. 5 and 6, asfollows:

First, the side faces (incidence and emergence surfaces) 11A of one ofthe half-assemblies 11 are placed on the oblique surfaces 17A and 19A ofthe abutment surface member 17 and the supporting surface member 19. Theprism 3-2, having the smaller thickness t₂, is located on the abutmentsurface member 17, so that the side face 11A thereof is located on theoblique surface 17A, and the stepped portions 5 (joining surfaces 3A)are located on the vertical reference surfaces 17C. This state isstable. Namely, the half-assembly 11 is firmly held in a stable positiondefined by the oblique surfaces 17A and 19A and the vertical referencesurfaces 17C.

Thereafter, an epoxy resin adhesive or an ultraviolet hardening adhesiveis applied to the upper joining surfaces 11C of the two prisms of thehalf-assembly 11, and then the other half-assembly 11 is placed on theupper joining surfaces 11C in such a way that the lower joining surfaces11C of the two prisms of the other half-assembly 11 are located on theupper joining surfaces 11C. Note that the prism 3-1 of a largerthickness t₁ is located on the abutment surface member 17. Namely, thewide prism 3-1 of the upper half-assembly 11 is located on the narrowprism 3-2 of the lower half-assembly 11.

Thereafter, the movable support 21 is moved forward until it comes intocontact with the stop 23 while the vertical reference surface 21C of themovable support 21 bears against the stepped portions 5 of the upperhalf-assembly 11.

When the movable support 21 comes into contact with the stop 23, thevertical reference surfaces 21C and 17C lie on a same plane, asmentioned above, and accordingly, the stepped portions 5 (joiningsurfaces 3A) of the upper and lower half-assemblies 11, on which thevertical reference surfaces 21C and 17C bear, also lie on a same plane.Consequently, the upper half-assembly can be firmly joined to the lowerhalf-assembly by adhering the side faces 11C of the half-assemblies toeach other by an epoxy adhesive or ultraviolet hardening adhesive tocomplete a prism assembly. Thus, according to the present invention, arectangular parallelepiped prism 1 can be precisely and easily obtainedwithout using an additional optical instrument, such as a microscope.

It goes without saying that when a dichroic prism as shown in FIG. 7 isprepared according to the present invention, predetermined reflectingfilms are interposed between the joining surfaces.

Although the prisms 3-1, having a larger thickness t₁, and the prisms3-2, having a small thickness t₂, are alternately arranged to form arectangular parallelepiped prism in the illustrated embodiment, it ispossible to manufacture a rectangular parallelpiped prism which has tworight prisms 3-1 of a larger thickness t₁, and two right prisms 3-2 of asmall thickness t₂ that are located side by side. Namely, the rightprism 3-1 of the upper half-assembly is located on the right prism 3-1of the lower half-assembly, and the right prism 3-2 of the upperhalf-assembly is located on the right prism 3-1 of the lowerhalf-assembly. In this alternative, the direction of the movable support21 is reversed. Namely, the arms 21B of the movable support 21 projectin the right hand direction in FIG. 6. That is, the movable support 21in the alternative and the movable support shown in FIG. 6 aresymmetrical with respect to the vertical plane in which the referencesurfaces 17C lie.

Although in the illustrated embodiment, the prisms 3-1 and 3-2 havedifferent thicknesses to provide stepped portions defining thepositioning reference surfaces in the above-mentioned embodiment, thethicknesses of the prisms are optional. The significant concept of thepresent invention is the provision of a partially exposed referencesurface on the joining surfaces of two prisms when the latter areadhered to each other.

For instance, if the two prisms 3-1 and 3-2 have the same thickness, thejoining surfaces thereof can be partially deviated from the centerthereof in the direction of the thickness to provide an exposedreference surface.

The prisms are not limited to the right prisms and can be of anysectional shape, such as trapezoid.

Furthermore, the number of prisms is not limited to four and isoptional.

Although all the prisms are assembled in such a way that they areparallel with each other in the direction of the thickness in theillustrated embodiment, the parallel arrangement is not alwaysnecessary. For instance, it is possible to connect the twohalf-assemblies at a predetermined angle in the direction of thethickness.

I claim:
 1. An apparatus for manufacturing a joined prism assemblyhaving a plurality of prism half-assemblies, each having more than oneprism adhered to each other at joining surfaces, and having at least oneexposed reference surface for positioning, wherein said apparatuscomprises:prism supporting means for holding one of said prismhalf-assemblies at a predetermined position, including first referencesurfaces with which outer surfaces of said prism half-assembly can comeinto contact and at least one second reference surface with which anexposed reference surface can come into contact, positioning means forpositioning another prism half-assembly which is located on and adheredto said one prism half-assembly held by said prism supporting means,said positioning means provided with a third reference surface having apredetermined relationship to said second reference surface, and saidprism assembly being in the shape of a rectangular parallelpiped andincludes two wide and two narrow right-angle prisms having an isoscelesright triangle cross-sectional shape and adhered to each other at theiradjacent sides defining right-angle apexes, each of said prismhalf-assemblies having at least one stepped portion between end faces ofthe prisms adjacent to said joining surfaces for defining at least oneexposed positioning reference surface when said prisms are adhered toeach other.
 2. The apparatus according to claim 1, wherein saidpositioning means positions said other prism half-assembly on said oneprism half-assembly held on said prism supporting means, with the helpof said reference surface of said other prism half-assembly.
 3. Theapparatus according to claim 1, wherein said prism supporting meanscomprises a pair of oblique surfaces with which adjacent side faces ofsaid one prism half-assembly defining one of said right angle apexesthereof can come into contact and at least one vertical referencesurface with which said at least one exposed reference surface of saidprism half-assembly can come into contact.
 4. The apparatus according toclaim 1, wherein said positioning means comprises said at least onevertical reference surface which lies in the same vertical plane as saidat least one vertical reference surface of said prism supporting means.5. The apparatus according to claim 4, wherein said at least onevertical reference surface of said positioning means faces in adirection opposite to said at least one vertical reference surface ofsaid prism supporting means.
 6. The apparatus according to claim 4,wherein said at least one vertical reference surface of said positioningmeans faces in the same direction as said at least one verticalreference surface of said prism supporting means.
 7. The apparatusaccording to claim 5, wherein said positioning means comprises a movablemember on which said at least one vertical reference surface isprovided.
 8. The apparatus according to claim 6, wherein saidpositioning means comprises a movable member on which said at least onevertical reference surface is provided.
 9. The apparatus according toclaim 7, further comprising a base plate on which said movable member ismovably supported.
 10. The apparatus according to claim 8, furthercomprising a base plate on which said movable member is movablysupported.
 11. The apparatus according to claim 9, further comprising astop for restricting the movement of said movable plate on said baseplate.
 12. The apparatus according to claim 10, further comprising astop for restricting the movement of said movable plate on said baseplate.